Unlike the modern Bessemer, open-hearth, and other steel products which are reworked, i.e., rolled, forged, etc., cast iron is a comparatively old alloy dating back over several centuries. It cannot be rolled, forged, or otherwise reshaped, so its final form must be given to it at once by pouring or “casting” the molten metal into a mold. Its castings serve exceedingly well in the hundreds of places for which they are adapted. They are comparatively cheap, can be readily duplicated in small or large quantities, and those from the softer grades of cast iron may be machined easily.
These cast iron alloys have only from one-third to one-half the strength of steel or wrought iron and are, comparatively speaking, very brittle. Where resistance to severe shock must be withstood they should not be used. Also, some varieties have a “habit” of growing larger upon repeated heating and cooling. This “permanent growth” is particularly noticeable when the alternate heating and cooling is at red heat or over. Pieces of cast iron have been made to gain 15 per cent in linear dimensions, and it is quite common knowledge among machinists that a piece of cast iron which is slightly too small can be permanently expanded by heat.
Nevertheless, the cast irons have large and legitimate fields in which they are very serviceable. From most of their important present uses they are not likely soon to be displaced.
Because of Their Large Size Molds for Very Large Castings Have to Be Made on the Floor of the Foundry, or Partly in Pits
Cast irons in considerable variety of compositions and physical properties are available, as was indicated by alloys Nos. 14 to 19 which were given in the table on page [83], part of which is here reproduced. In alloys 3 to 13 the carbon exerts the great influence on the physical properties, and this is true also of the cast irons. But all of the latter have a total carbon content of more than 2½ per cent, and, under certain conditions, some of the carbon assumes a different form from that which we encountered in the steels. This modified form is “graphite,” well known to us as a flaky, black, greasy-feeling material, which is soft and very fragile. Graphite in the iron alloy naturally weakens it and, as it is itself such a good lubricant, it makes cast iron machine easily if sufficient amount is present.
| A Few of the Cast Irons | |||||
|---|---|---|---|---|---|
| Silicon, Per Cent | Graphite, Per Cent | Combined Carbon, Per Cent | Total Carbon, Per Cent | ||
| No. 14. White Cast Iron | .70 | .10 | 2.65 | 2.75 | Very Hard |
| No. 15. Annealed Malleable Iron | .70 | 2.70 | .05 | 2.75 | Machinable |
| No. 16. Cast Iron for Chilled Castings | 1.00 | 1.00 | 2.00 | 3.00 | Very Hard |
| No. 17. Semi-steel | 1.75 | 2.80 | .40 | 3.20 | Machinable |
| No. 18. Gray Cast Iron | 2.00 | 3.10 | .30 | 3.40 | Machinable |
| No. 19. Soft Gray Cast Iron | 2.50 | 3.30 | .15 | 3.45 | Machinable |
Now, the above must be understood as being typical compositions only. There are, of course, irons of all intermediate compositions, also, and while the total, graphitic and combined carbons, typically, are about as indicated, there may be wide variation.
To illustrate what a variety of chemical and physical properties may be produced, let us assume that the total carbon in a certain cast iron is 3.25 per cent. If this carbon is all in the chemically combined form (i. e., combined with the iron to form the very hard compound which is known to the metallographist as “cementite”) the fracture will be white and the alloy extremely hard. If none of this carbon is combined, but all is in the form of graphite flakes throughout the alloy, the fracture will be “gray” and the alloy soft and machinable. It is possible to produce either of these two conditions or practically any intermediate stage; i.e., we can almost at will split up the 3.25 per cent of carbon into varying percentages of graphitic and combined carbon—the total always equaling 3.25 per cent.
